Well, no, actually, Bob Dylan is not an objectively bad vocalist, in my opinion. Hear me out, and I'm sorry if this looks like splitting hairs or being pedantic, but...
Maybe an objectively bad singer, depending on the situation. He's not in an academic setting trying to pay attention to the dynamics, pitch, articulations etc. of some composer in a classical setting, he's not being graded or listening to a coach. But he's out there singing his own songs, stuff he made himself, and loads of people like his voice. You can't call him objectively bad, because that is his art and he's performing it his way. That's the beauty of art. Dylan ain't under the illusion that he's some great singer, but he's not trying to be, that's not his goal. He's not doing opera. I don't like Dylan at all but I don't agree with calling him an objectively bad vocalist (I think there is a difference in that term compared with 'singer' as anal as it might seem). Same with Tom Waits after his first album.